Industrial IoT Authors: Gordon Haff, Elizabeth White, Stackify Blog, Yeshim Deniz, SmartBear Blog

Related Topics: Java IoT, Industrial IoT, Mobile IoT, Microservices Expo, IBM Cloud, Weblogic, Machine Learning

Java IoT: Article

Component Models in Java | Part 2

OSGi Component Model

OSGi is the latest component model to join the bandwagon of component models, which provides a platform for component oriented development and assembly. OSGi framework is a standards based platform whose specifications are provided by the OSGi Alliance (www.osgi.org, formerly OSGi was referred as Open Services Gateway Initiative). OSGi Alliance is an industry backed nonprofit organization which was founded in March 1999. The OSGi specification has gone through many releases and the current major version in use is 4 and version 5 has been introduced recently.

The OSGi defines a dynamic module system for Java. This offers help for Java's modularity problems by providing better control to the code structure, manage the lifecycle of the code and a complete loosely coupled approach that is much needed for component-oriented development.

The OSGi specification consists of two parts:

  • OSGi Framework
  • OSGi Standard services

The OSGi framework is the OSGi runtime environment that provides all the functionality as per the specifications. Applications are deployed and executed in the OSGi framework. The OSGi framework provides an API for the development of components. There are a number of framework implementations and some of the popular ones are Eclipse Equinox, Apache Felix and Knoplerfish. OSGi standard services define reusable services that should be provided as part of the development platform implementation. There are three conceptual layers in OSGi framework:

  • Module layer - Responsible for packaging and sharing code
  • Lifecycle layer - Responsible for managing the lifecycle of deployed module during runtime
  • Service layer - Responsible for dynamic service publication, searching and binding

OSGi Bundle
An OSGi bundle is a deployment module in the form of a JAR file. A module in OSGi parlance is known as a bundle. Bundles contain class files and resource files, similar to the regular JAR file in Java, but in addition they contain manifest information that contains metadata about the bundle. Apart from the regular JAR file's manifest contents, a bundle's manifest file has OSGi specific information such as module name, version number, dependencies, etc., thus giving better modularity and easy maintainability. Bundles are more powerful than JAR files in enforcing module boundaries, because a bundle needs to explicitly define what portion of its internal code is externally visible. Similarly, a bundle must explicitly declare any external dependencies that it has with the code exposed by other bundles. A bundle must have a unique identity - Bundle Name and Version.

The OSGi framework matches the exports and imports of deployed bundles to dynamically wire the entire application. This process of bundle resolution ensures consistency among the different bundles in terms of versions and other constraints. An application in OSGi is nothing but a collection of bundles with explicitly defined dependencies. A bundle is deployed in OSGi framework once it is developed.

OSGi Service Registry
The OSGi Service registry promotes service oriented programming. The service registry provides service publication service discovery and service binding. The bundles deployed in the OSGi framework can leverage the service registry later for publishing and consuming services. A bundle providing a service publishes the service in the OSGi Service Registry. A service is defined by a Java Interface, which represents a conceptual contract between the provider and consumer. A potential consumer can use the registry to search for providers of a particular service. Once if finds a service provider, it can bind and use the service. Services layer in OSGi facilitates one more level of dynamism other than bundles. Just as bundles can be added and removed in a running application, the Services can appear and go dynamically in a runtime application.

OSGi Component
As discussed earlier, a bundle is the deployment unit in OSGi component model. A bundle is a JAR file that contains:

  • Class files
  • Resource files
  • Manifest file (with additional metadata)

The class files are typically the interface and the implementation which constitutes the component. The manifest will have additional metadata as shown below:

Manifest-Version: 1.0

Bundle-ManifestVersion: 2

Bundle-Name: com.demo.helloWorld

Bundle-SymbolicName: com.demo.helloWorld

Bundle-Version: 1.0.0.qualifier

Bundle-Activator: com.demo.Activator

Bundle-Vendor: PIRAM

Bundle-RequiredExecutionEnvironment: JavaSE-1.7

Import-Package: org.osgi.framework;version="1.3.0"

Bundle-ActivationPolicy: lazy

The OSGi Framework provides an inbuilt API called BundleActivator which helps the bundle to hook its own lifecycle management. The BundleActivator interface has two methods - start() and stop() which are invoked when the bundle is started and stopped respectively. Any bundle can implement this interface to check its own life cycle. The bundle could perform actions as specified in the start and stop methods of the Activator class. The use of bundle as a component for building application on the OSGi framework does not just depend on the bundle doing the work whenever it is started or stopped. The bundle needs to be able to expose certain functionality as provided interfaces and it needs to consume functionalities as per the require interfaces. Thus a collection of bundles made into an assembly should be able to work together to form a system. Generally the provided interface will be created as a separate bundle and the implementations can be wired dynamically by the OSGi runtime from the implementation bundles. There can be more than one implementation, the wiring happens depending on the runtime.

Example to understand OSGi Component Model
The OSGi component model can be understood with the same shopping Cart example discussed in the earlier models.

Figure 3: OSGi Component Model - Cart Component Example

The Cart application in this example is created with the following bundles for better modularity and maintainability.

  1. Interface Bundle (com.online.shopping)
  2. Implementation Bundle (CartImpl)
  3. Client Bundle (CartClient)

The Cart component is comprised of interface bundle and implementation bundle. The interface bundle (com.online.shopping) defines an interface ICart. This interface will be used by the implementation bundle to invoke the exposed services. The client bundle will use the interface for invoking the required services which gets bounded to the implementation bundle by the service registry.

Interface Bundle
The interface bundle contains the interface ICart for the Cart component and is defined as below:

package com.online.shopping;

import java.util.Collection;

public interface ICart {
public void addItem(Product product, int quantity);
public Collection<Product> listItems();    
public double getTotalPrice();
public void clearCart();

This bundle has ONLY the interface and its helper class and it exports the com.online.shopping package as shown in the manifest file below:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: CartIntf
Bundle-SymbolicName: CartIntf
Bundle-Version: 1.0.0.qualifier
Bundle-ActivationPolicy: lazy
Bundle-RequiredExecutionEnvironment: JavaSE-1.6
Import-Package: org.osgi.framework;version="1.3.0"
Export-Package: com.online.shopping

The structure of the bundle jar file is as below:

Figure 4: Structure of Interface Bundle

Implementation Bundle
ICart interface is implemented by the class CartImpl, whose code as demonstrated below:

package com.online.shopping.impl;

import java.util.Collection;
import java.util.HashMap;
import java.util.Map;

import com.online.shopping.ICart;
import com.online.shopping.Product;

public class CartImpl implements ICart {
Map<Product, Integer> items = new HashMap<Product, Integer>();      

public void addItem(Product product, int quantity) {
if(items.containsKey(product)) {
quantity +=items.get(product);
items.put(product, quantity);


public Collection<Product> listItems() {

return items.keySet();


public double getTotalPrice() {
double totalPrice = 0;
for(Product product: items.keySet()) {
totalPrice+=product.getPrice()* items.get(product);           
return totalPrice;

public void clearCart() {


The CartImpl is the class in the implementation bundle that implements the ICart interface and provides the ICart service implementation. In the implementation bundle, the interface com.online.shopping.ICart is not added to the CLASSPATH, but imported by the OSGi framework. This bundle imports the interface bundle as explained in the MANIFEST.MF below:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: CartImpl
Bundle-SymbolicName: CartImpl
Bundle-Version: 1.0.0.qualifier
Bundle-RequiredExecutionEnvironment: JavaSE-1.6
Import-Package: com.online.shopping,
Service-Component: META-INF/component.xml

The implementation bundle is exposed as a declarative service component. From the manifest file, it is evident that the bundle is not exported as a package, but it is exposed as a service with the entry - Service-Component that this is exposed as a component and the component description is available in component.xml. With the help of such XML files, components declare their provided services. The OSGi framework helps to publish the CartImpl as a service in the OSGi service registry. The component.xml is as below:

<?xml version="1.0" encoding="UTF-8"?>
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" name="CartImpl">
<implementation class="com.online.shopping.impl.CartImpl"/>
<provide interface="com.online.shopping.ICart"/>

The ICart is exposed as a service and the service is implemented by the CartImpl implementation class. Looking at the component.xml, it is clear that the component provides the ICart service. The component declares the implementation class and the provided interface. The declarative services in the OSGi framework publish the service at the execution time after the bundle is activated. The structure of the JAR file of the bundle is as follows:

Figure 5: Structure of Implementation Bundle

Client Bundle
The client bundle is supposed to consume the services exposed by the ICart service implementation and consume it. The client bundle is another component that imports the com.online.shopping package and consumes the service through OSGi service registry. The client bundle's manifest looks as below:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: CartClient
Bundle-SymbolicName: CartClient
Bundle-Version: 1.0.0.qualifier
Bundle-RequiredExecutionEnvironment: JavaSE-1.6
Import-Package: com.online.shopping,
Service-Component: META-INF/component.xml

The client is also a component which consumes the services provided by the ICart component.

Figure 6: Structure of Client Bundle

The component.xml in client bundle has reference to the ICart service interface.

<?xml version="1.0" encoding="UTF-8"?>
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" name="CartClient">
<implementation class="com.client.CartClient"/>
<reference bind="gotService" cardinality="1..1" interface="com.online.shopping.ICart" name="ICart" policy="dynamic" unbind="lostService"/>

Apart from the interface reference, the component.xml also refers to some methods called ‘gotService' and ‘lostService' during binding and unbinding of service references. These are the methods defined in the client class which will be invoked with the associated service references into the service object. This allows the component to find out the services without retrieving them. The declarative specifications in OSGi framework defines the methods where the service reference will be injected. The component service policy may be static or dynamic. In static policy, the service reference is injected once and not changed until the component is deactivated. Where as in the dynamic policy, the component is notified whenever the service comes or goes utilizing the true dynamism. In the example, it is dynamic. The client invokes the ICart service as follows:

package com.client;

import java.util.Collection;

import org.osgi.framework.ServiceReference;
import org.osgi.service.component.ComponentContext;

import com.online.shopping.ICart;
import com.online.shopping.Product;

public class CartClient {

ComponentContext context;
ServiceReference reference;
ICart cart;

public void activate(ComponentContext context) {
System.out.println("Activate Component");

if(reference!= null) {
cart = (ICart)context.locateService("ICart", reference);

Product product = new Product();
cart.addItem(product, 20);

Product newProduct = new Product();
newProduct.setName("Enterprise OSGi");
cart.addItem(newProduct, 10);

Collection<Product> productItems = cart.listItems();
for(Product items: productItems) {
System.out.println(items.getName()+"******"+ items.getPrice());             

System.out.println("Total Price of Cart Items: "+cart.getTotalPrice());



public void gotService(ServiceReference reference) {
System.out.println("Bind Service");
this.reference = reference;

public void lostService(ServiceReference reference) {
System.out.println("unbind Service");
this.reference = null;           


The client has defined three methods:

  • activate - part of declarative services API. This method is invoked when this component is activated. The ComponentContext is used to locate the ICart with the injected service reference.
  • gotService - user defined method as available in the component.xml, this method is invoked with the service reference (using dependency injection) when the service object is binded.
  • lostService - user defined method as mentioned in the client component.xml, this method is invoked with the injected service reference when the service object is unbinded.

Figure 7: Cart Component Bundles Deployment in OSGi Container

The client is not even aware of the implementation bundle. If there are multiple implementations available for the same service, the service is bounded dynamically by the environment. If there is any change in the implementation, only the implementation bundle will undergo change. A revised bundle can provide additional services which can be consumed by clients. So replacing components is easier and will not affect any other component. This way, OSGi gives good modularity by de-coupling components and a pluggable dynamic service model which are much needed features of a component model.

More Stories By Piram Manickam

Piram Manickam works at Infosys Limited. He would like to acknowledge and thank Sangeetha S, a beloved colleague and friend, for her invaluable contributions in this work.

More Stories By Subrahmanya SV

Subrahmanya SV works at Infosys Limited. He would like to acknowledge and thank Sangeetha S, a beloved colleague and friend, for her invaluable contributions in this work.

More Stories By S Sangeetha

S Sangeetha is a Senior Technical Architect at the E-Commerce Research Labs at Infosys Limited. She has over 15 years of experience in architecture, design and development of enterprise Java applications. She is also involved in enhancing the technical skills of Architects at Infosys. She has co-authored a book on ‘J2EE Architecture’ and also has written numerous articles on Java for various online Java forums like JavaWorld, java.net, DevX.com and internet.com. She can be reached at [email protected]

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.

@ThingsExpo Stories
Enterprises have taken advantage of IoT to achieve important revenue and cost advantages. What is less apparent is how incumbent enterprises operating at scale have, following success with IoT, built analytic, operations management and software development capabilities - ranging from autonomous vehicles to manageable robotics installations. They have embraced these capabilities as if they were Silicon Valley startups.
The standardization of container runtimes and images has sparked the creation of an almost overwhelming number of new open source projects that build on and otherwise work with these specifications. Of course, there's Kubernetes, which orchestrates and manages collections of containers. It was one of the first and best-known examples of projects that make containers truly useful for production use. However, more recently, the container ecosystem has truly exploded. A service mesh like Istio addr...
Predicting the future has never been more challenging - not because of the lack of data but because of the flood of ungoverned and risk laden information. Microsoft states that 2.5 exabytes of data are created every day. Expectations and reliance on data are being pushed to the limits, as demands around hybrid options continue to grow.
Business professionals no longer wonder if they'll migrate to the cloud; it's now a matter of when. The cloud environment has proved to be a major force in transitioning to an agile business model that enables quick decisions and fast implementation that solidify customer relationships. And when the cloud is combined with the power of cognitive computing, it drives innovation and transformation that achieves astounding competitive advantage.
Poor data quality and analytics drive down business value. In fact, Gartner estimated that the average financial impact of poor data quality on organizations is $9.7 million per year. But bad data is much more than a cost center. By eroding trust in information, analytics and the business decisions based on these, it is a serious impediment to digital transformation.
Digital Transformation: Preparing Cloud & IoT Security for the Age of Artificial Intelligence. As automation and artificial intelligence (AI) power solution development and delivery, many businesses need to build backend cloud capabilities. Well-poised organizations, marketing smart devices with AI and BlockChain capabilities prepare to refine compliance and regulatory capabilities in 2018. Volumes of health, financial, technical and privacy data, along with tightening compliance requirements by...
Andrew Keys is Co-Founder of ConsenSys Enterprise. He comes to ConsenSys Enterprise with capital markets, technology and entrepreneurial experience. Previously, he worked for UBS investment bank in equities analysis. Later, he was responsible for the creation and distribution of life settlement products to hedge funds and investment banks. After, he co-founded a revenue cycle management company where he learned about Bitcoin and eventually Ethereal. Andrew's role at ConsenSys Enterprise is a mul...
DXWordEXPO New York 2018, colocated with CloudEXPO New York 2018 will be held November 11-13, 2018, in New York City and will bring together Cloud Computing, FinTech and Blockchain, Digital Transformation, Big Data, Internet of Things, DevOps, AI, Machine Learning and WebRTC to one location.
DXWorldEXPO | CloudEXPO are the world's most influential, independent events where Cloud Computing was coined and where technology buyers and vendors meet to experience and discuss the big picture of Digital Transformation and all of the strategies, tactics, and tools they need to realize their goals. Sponsors of DXWorldEXPO | CloudEXPO benefit from unmatched branding, profile building and lead generation opportunities.
DXWorldEXPO LLC announced today that "Miami Blockchain Event by FinTechEXPO" has announced that its Call for Papers is now open. The two-day event will present 20 top Blockchain experts. All speaking inquiries which covers the following information can be submitted by email to [email protected] Financial enterprises in New York City, London, Singapore, and other world financial capitals are embracing a new generation of smart, automated FinTech that eliminates many cumbersome, slow, and expe...
The best way to leverage your Cloud Expo presence as a sponsor and exhibitor is to plan your news announcements around our events. The press covering Cloud Expo and @ThingsExpo will have access to these releases and will amplify your news announcements. More than two dozen Cloud companies either set deals at our shows or have announced their mergers and acquisitions at Cloud Expo. Product announcements during our show provide your company with the most reach through our targeted audiences.
DevOpsSummit New York 2018, colocated with CloudEXPO | DXWorldEXPO New York 2018 will be held November 11-13, 2018, in New York City. Digital Transformation (DX) is a major focus with the introduction of DXWorldEXPO within the program. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of bus...
With 10 simultaneous tracks, keynotes, general sessions and targeted breakout classes, @CloudEXPO and DXWorldEXPO are two of the most important technology events of the year. Since its launch over eight years ago, @CloudEXPO and DXWorldEXPO have presented a rock star faculty as well as showcased hundreds of sponsors and exhibitors! In this blog post, we provide 7 tips on how, as part of our world-class faculty, you can deliver one of the most popular sessions at our events. But before reading...
Cloud Expo | DXWorld Expo have announced the conference tracks for Cloud Expo 2018. Cloud Expo will be held June 5-7, 2018, at the Javits Center in New York City, and November 6-8, 2018, at the Santa Clara Convention Center, Santa Clara, CA. Digital Transformation (DX) is a major focus with the introduction of DX Expo within the program. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive ov...
As IoT continues to increase momentum, so does the associated risk. Secure Device Lifecycle Management (DLM) is ranked as one of the most important technology areas of IoT. Driving this trend is the realization that secure support for IoT devices provides companies the ability to deliver high-quality, reliable, secure offerings faster, create new revenue streams, and reduce support costs, all while building a competitive advantage in their markets. In this session, we will use customer use cases...
DXWorldEXPO LLC announced today that ICOHOLDER named "Media Sponsor" of Miami Blockchain Event by FinTechEXPO. ICOHOLDER give you detailed information and help the community to invest in the trusty projects. Miami Blockchain Event by FinTechEXPO has opened its Call for Papers. The two-day event will present 20 top Blockchain experts. All speaking inquiries which covers the following information can be submitted by email to [email protected] Miami Blockchain Event by FinTechEXPO also offers s...
With tough new regulations coming to Europe on data privacy in May 2018, Calligo will explain why in reality the effect is global and transforms how you consider critical data. EU GDPR fundamentally rewrites the rules for cloud, Big Data and IoT. In his session at 21st Cloud Expo, Adam Ryan, Vice President and General Manager EMEA at Calligo, examined the regulations and provided insight on how it affects technology, challenges the established rules and will usher in new levels of diligence arou...
Dion Hinchcliffe is an internationally recognized digital expert, bestselling book author, frequent keynote speaker, analyst, futurist, and transformation expert based in Washington, DC. He is currently Chief Strategy Officer at the industry-leading digital strategy and online community solutions firm, 7Summits.
Digital Transformation and Disruption, Amazon Style - What You Can Learn. Chris Kocher is a co-founder of Grey Heron, a management and strategic marketing consulting firm. He has 25+ years in both strategic and hands-on operating experience helping executives and investors build revenues and shareholder value. He has consulted with over 130 companies on innovating with new business models, product strategies and monetization. Chris has held management positions at HP and Symantec in addition to ...
Cloud-enabled transformation has evolved from cost saving measure to business innovation strategy -- one that combines the cloud with cognitive capabilities to drive market disruption. Learn how you can achieve the insight and agility you need to gain a competitive advantage. Industry-acclaimed CTO and cloud expert, Shankar Kalyana presents. Only the most exceptional IBMers are appointed with the rare distinction of IBM Fellow, the highest technical honor in the company. Shankar has also receive...